Chromium is a metal used in a variety of industries such as tanneries, metallurgy and metal electroplating. Although chromium oxidation states range from (−IV) to (+VI), only the (+III) and (+VI) are stable in the environment. Chromium (VI) exists as oxyanions of chromate at pH higher than 6.5 and as dichromate at pH below 6.5. Chromium (VI) is very soluble in water. However, chromium (III) has a very lower solubility in water and readily precipitates as Cr(OH)3 or as mixed Fe(III)-Cr(III) (oxy)hydroxides under alkaline or slightly acidic conditions. Chemical reduction of chromium (VI) to chromium (III) followed by precipitation, adsorption, ion exchange and membrane processes are the most used techniques at an industrial scale for the decontamination of chromium bearing solution containing chromium (VI) ions. Recent methods disclosed the reduction of chromium (VI) to chromium (III) using scrap iron fibres.
However, none of these methods provide a yield and a rate of chromium (VI) reduction high enough to be applicable at an industrial scale.
The problems with these methods are: The kinetics of the reduction of chromium (VI) to chromium (III) is slow under the conditions described therein; The chromium (III) produced from the reduction of chromium (VI) does not precipitate in the exchange mass because of the lack of process control, especially the pH adjustment to specific values; and the RedOx reaction between iron particles from the iron fibres and the chromium (VI) available in the chromium bearing solution is not controlled.